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Effects of By-Catch Reduction Devices (BRDs) on Commercial Crab Catch
Will BennettUniversity of Mary Washington
Mentors: Romuald Lipcius, Rochelle Seitz, and Danielle McCulloch
August 4, 2010
Introduction• Blue crab industry– $25 Million+ in 2008
• Terrapin mortality– Other species
• Pot types• BRDs• Rook et al. 2010• Opposition- Fishing Industry• Watermen Claims– Catch increase in peeler pots– Catch decrease in standard pots
Objectives
• Determine if BRD usage has any impact on commercial crab catch
• Determine scientifically if the BRD claims of the watermen have any ground
• Make a policy recommendation to the VMRC about BRD usage in commercial crabbing
Expected Results
• Hypotheses-– BRD usage will decrease crab catch in standard
crab pots– BRD usage will increase crab catch in peeler crab
pots– BRD usage will decrease bycatch in both peeler
and standard crab pots– Crabs caught in BRD pots will have lesser widths
than those in non BRD pots
Methods• Three rivers• Upriver/Downriver location• Peeler Pots– 20 per location
• Standard Pots– 4 Per location
Up Rapp.
Down Rapp.
Up York
Down York
Up James
Down James
Statistics
• Statistical method– AIC statistics
• Ranks the models given by best fit to the data
– Use 3 models to test the response variables, crab catch and total bycatch number, against BRD usage, river, and upriver/downriver location• Model 1 (BRD)
– y = β0+β1X1+ξ
• Model 2 (Location + River)– y = β0+β2X2+β3X3+β4X4+ξ
• Model 3 (BRD + Location + River)– y = β0+β1X1+β2X2+β3X3+β4X4+ξ
• BRD = β1, Upriver/downriver= β2, River= β3 and β4,
Legal Crab Catch
NO BRD BRD0
0.5
1
1.5
2
2.5
3
3.5
4
Mean Crab Catch by BRD + SE – Standard Pot
BRD
Lega
l cra
bs P
er P
ot
NO BRD BRD0
0.5
1
1.5
2
2.5
3
3.5
Mean Crab Catch by BRD + SE – Peeler Pot
BRD
Lega
l Cra
bs P
er P
ot
• ~0.5 per pot more without BRD• Little confidence with SE
•~1 per pot more without BRD•Strong confidence
Crab Catch AIC TablesModel- Stan.
AIC Model Weight
α β1 BRD β2 Loc-upper
β3 R v J β4 Y v J
BRD 741.66 <0.005 3.55±0.31 -0.60±0.44 Na Na Na
Loc + River
708.56 0.52 3.10±0.43 Na -2.84±0.44 1.75±0.57 1.41±.57
BRD + Loc + River
708.71 0.48 3.38±0.47 -0.54±0.39 -2.84±0.43 1.73±0.57 1.42±.57
Model- Peel.
AIC Model Weight
α β1 BRD β2 Loc-upper
β3 R v J β4 Y v J
BRD 2342.98 <0.005 2.38±0.13 0.92±0.18 Na Na Na
Loc + River
2291.96 <0.005 3.03±0.18 Na -1.34±0.18 1.17±0.22 0.05±0.21
BRD + Loc + River
2262.04 0.99 2.57±0.19 0.93±0.16 -1.36±0.17 1.15±0.21 0.03±0.20
Bycatch
NO BRD BRD0
0.05
0.1
0.15
0.2
0.25
0.3
Mean Bycatch by BRD Use + SE – Peeler Pots
BRD
Byca
tch
Indi
vidu
als P
er P
ot
•The primary bycatch species were Croaker, Oystertoad, Flounder, Perch, Spot, Hogchoker, Catfish, and Eel
NO BRD BRD0
0.05
0.1
0.15
0.2
0.25
0.3
Mean Bycatch by BRD Use + SE – Standard Pots
BRD
Byca
tch
Indi
vidu
als P
er P
ot
•Only four Terrapins were caught in this study•All in non-BRD pots
Bycatch AIC TablesModelStan.
AIC Model Weight
α β1 BRD β2 Loc-upper
β3 R v J β4 Y v J
BRD 315.3 0.03 0.29±0.07 -0.09±0.1 Na Na Na
Loc + River
309.46 0.59 0.09±0.11 Na -0.12±0.11 0.08±0.15 0.4±0.15
BRD + Loc + River
310.33 0.38 0.15±0.12 -0.11±0.1 -0.12±0.11 0.08±0.15 0.4±0.15
ModelPeel.
AIC Model Weight
α β1 BRD β2 Loc-upper
β3 R v J β4 Y v J
BRD 1585.23 <0.005 0.46±0.06 0.56±0.09 Na Na Na
Loc + River
1571.13 <0.005 0.89±0.09 Na -0.39±0.09 0.46±0.11 -0.31±0.1
BRD + Loc + River
1529.3 1.0 0.61±0.1 0.56±0.08 -0.4±0.09 0.45±0.11 -0.31±0.1
Width•Minimal difference in crab carapace width when using a BRD•Standard Pot
BRD NO BRD142
143
144
145
146
Mean Carapace Width vs. BRD + SE
BRD
Cara
pace
Wid
th
Statistics – Crab Catch
• Standard Pots– Location + River
• Peeler Pots– BRD + Location + River
Statistics - Bycatch• Standard Pots– Location + River
• Peeler Pots– BRD + Location + River
Results
• Effects on crab catch– Not confident that BRDs affect standard pot crab catch– BRDs negatively affect peeler pot catch
• Consistent effects on bycatch– BRDs decrease bycatch in both peeler and standard
crab pots• AIC - River and location to explain bycatch and
crab catch in standard pots fit the data better than the same two with BRD included
Policy Discussion• Slightly fewer crabs with BRDs• Less bycatch• All 4 terrapins caught in non BRD pots• Judgment Call economically
and ethically
Acknowledgments
• Grant - Drs. Linda C. Schaffner and Rochelle D. Seitz NSF OCE 0552612
• Blue Crab Disaster Relief Funds from NOAA• Thanks to Romuald Lipcius, Rochelle Seitz, Danielle
McCulloch, Allison Colden, Gina Ralph, Mike Seebo, Alison Smith, Seth Theurkeuf, Emily Kimminau, Liz Gomez, Cassie Bradley, Gabby Saluta, Cassie Glaspie, Andrew 107 lab group, my fellow REU interns, and all the watermen who participated in this study
